In recent years, Japan started digital terrestrial broadcasting by ISDB-T (Integrated Services Digital Broadcasting-Terrestrial transmission), and Europe started digital terrestrial broadcasting by DVB-T (Digital Video Broadcasting-Terrestrial transmission). Furthermore, DVB-H (Digital Video Broadcasting-Handhelds transmission) for mobile devices is about to be launched.
These transmission methods employ OFDM which is one of the digital modulation schemes. OFDM has an excellent frequency use efficiency and is suitable for high-speed data communications.
Each symbol of an OFDM signal in ISDB-T is composed of an effective symbol duration signal and a guard interval signal. At the transmitting end, the effective symbol duration signal and the guard interval signal inserted before the effective symbol duration signal are transmitted. The effective symbol duration signal is a signal on a time axis generated as a result of modulation by performing an Inverse FFT (IFFT) on amplitude and phase information of each subcarrier on a frequency axis. The guard interval signal is a copy of a latter portion of the effective symbol duration signal.
At the receiving end, A/D conversion and quadrature detection are performed on the received OFDM signal, synchronization of each symbol is detected based on the quadrature detected signal, and a FFT is performed on a signal of an effective symbol length for each symbol, with it being possible to demodulate into amplitude and phase information on the frequency axis.
Patent document 1 discloses a technique of obtaining a proper FFT result based on a symbol structure of an OFDM signal by setting, even when intersymbol interference by multipath occurs in each received symbol, a start position of a FFT window so as to exclude a period during which the intersymbol interference occurs if that period is within the range of the guard interval.
However, other than multipath interference, impulse noise of an extremely large amplitude may be received depending on an OFDM signal reception environment.
If a FFT is performed on a symbol containing such noise for demodulation in the above manner, the impulse noise significantly influences the entire symbol, and original data cannot be recovered even by error correction after the demodulation.
Patent document 2 describes a technique of, when impulse noise of a large amplitude is received, setting an amplitude of a portion where the impulse noise is detected to 0 before performing a FFT, thereby reducing an adverse effect of the impulse noise.    Patent document 1: Japanese patent application publication No. 2002-171238.    Patent document 2: Japanese patent application publication No. 2002-152613.